Typically, in motor vehicles using hydrogen, compressed natural gas (CNG), or other gases used to power fuel cells or internal combustion engines, present practice is that fuel is stored in on board tanks maintained at a maximum pressure in the range of about 5000 psi for hydrogen and 3600 psi for CNG. Higher pressures in the range of about 10,000 psi or more are anticipated as the use of hydrogen and CNG becomes more prevalent and tank and compressors are developed for higher pressures. In situ techniques have been developed to manage thermal energy between high pressure gas in a tank and the environment of the tank. Advances in the art of thermal management systems for high pressure gas storage tanks are described in co-pending applications for United States Letters Patent of Kyoshi Handa assigned to Honda Motor Company, LTD: Gas Cooling Methods for High Pressure Fuel Storage Tanks on Vehicles Powered by Compressed Natural Gas or Hydrogen, Ser. No. 11/279,574 filed on Apr. 13, 2006; Pressure Powered Cooling System for Enhancing the Refill Speed and Capacity of On Board High Pressure Vehicle Gas Storage Tanks, Ser. No. 11/380,429 filed on Apr. 27, 2006; Gas Cooling Method Using a Melting/Solidifying Media for High Pressure Storage Tanks for Compressed Natural Gas or Hydrogen, Ser. No. 11/381,005 filed on May 1, 2006; and System for Enhancing the Efficiency of High Pressure Storage Tanks for Compressed Natural Gas or Hydrogen, Ser. No. 11/380,996 filed on May 1, 2006. In the general field of high pressure storage of industrial gases, stress phenomena occur as a result of compression during fill and refill, decompression during consumption and exhaustion of the gas, and changes in the ambient temperature of the environment in which the tank is located.
In the specification herein, reference to tanks adapted to hydrogen powered vehicles also correlates with the use of the invention with CNG (compressed natural gas) powered vehicles and high pressure storage tanks for other industrial gases. As illustrations, hydrogen is referred to in the specification and examples. “Hydrogen” is a term in most instances, however, intended to be interchangeable with CNG and industrial gases; all are referenced as a “gas” or “high pressure gas.” The present example of the invention was developed for use in hydrogen and natural gas powered motor vehicle tanks; however, broader application of the technology described herein may include any industrial gas stored under high pressure, including air; hence, the specification is not limited to transportation fuel. In addition to hydrogen fuel cell vehicles, high pressure storage tanks are also elements necessary in the hydrogen internal combustion engine field to which the invention is also applicable.
The present invention is adaptable for high pressure storage of any industrial gas. Although the system described herein was developed for specific application to hydrogen and CNG powered vehicles, there is no limitation inherent to the invention that prevents its use for any industrial pressure vessel filling application. When a high pressure gas storage vessel (or tank) is filled with hydrogen, the gas in the pressurized on storage tank may be characterized as including two forms of energy: chemical energy from the gas itself (consumed in powering the vehicle), and mechanical and thermal energy associated with the high pressure under which the gas is refueled and stored (referred to as Internal Energy, U, in thermodynamics).
The thermal energy associated with the gas translates into temperature fluctuations inside and outside the on board tank that, along with pressure variations, cause mechanical stress in the tank system as a result of the expansion and contraction of the physical components that comprise the tank and the in situ devices therein, such as referenced above, that manage thermal energy associated with introducing high pressure gas into a tank during a refill sequence. High pressure itself is also a stress inducing factor in tank components.
During a high pressure refueling process, the interior of the on board tanks (i.e., the refill gas) becomes heated as a result of gas compression (or the transfer of energy from the form of enthalpy, H, to internal energy, U) as the tank pressure increases and other refueling parameters that affect the process. After refueling, the tank interior temperature and pressure decrease slowly as heat is transferred to the environment through the wall of the tank or as the fuel gas is consumed during vehicle operation. The attainment of a full refill tank pressure generally requires compensation to offset the temperature increase during the course of refueling. In one variation, the charge of fuel input into and stored in the tank, measured by pressure, is initially in excess of the optimum design tank pressure; in another, the gas is pre cooled; in yet another, a slow fill rate is employed. In all instances, mechanical stresses will be induced in the tank components. Relative axial displacement or rotation between an in situ heat exchanger or other thermal management device and a high pressure on board fuel storage tank occurs as a result of stress factors associated with temperature and pressure changes occurring in the tank assembly during fill and exhaustion of gas in the tank. A mechanical compensation system is therefore desirable for stresses induced within the tank and the environment of the tank.
Usually, fill pressures are at 5000 psi or lower. As pressures exceed 3600 psi and 5000 psi, and approach or exceed 10,000 psi, gas cooling, through in situ devices, becomes an important factor in providing a full tank refill in a decreased period of time in the refueling process; and with higher pressures, compensation for thermal and pressure induced stresses assumes more importance. Consequently, there exists a need for systems to manage the physical stresses induced by high pressure fill and exhaustion of on board tanks.